Method for shielding a gas effluent



l 1970 J. E. JACKSON 3,526,362

METHOD FOR SHIELDING A GAS EFFLUENT Original Filed Jan. 16, 1968 5 SheetsSheet 1 ATTORNEY Sept. 1, 1970 Original Filed Jan. 16., 1968 PERCENT OXYGEN J. E. JACKSON 3,526,362

METHOD FOR SHIELDING A GAS EFFLUENT 5 Sheets-Sheet 2 I] ARGON 0.8 m ANNULU'S h. Q ARGON 3.2m ANNULUS HELIUM 0.8 m ANNULUS o 400 aoo I200 I600 2000 FLOW RATE, CFH

INFLUENCE OF PROCESS PARAMETERS 0N OXYGEN CONCENTRATION.

FIG. 2

INVENTOR. JOHN E. JACKSON ATTORNEY Sept. 1, 1970 Original Filed Jan. 16, 1968 PERCENT OXYGEN AIR J. E. JACKSON ms'rnonr'on snmwms A GAS lmmursmv 5 Sheets-Sheet 5 E] ARGON 0.8 m ANNULUS Q ARGQN 3.3INZ'ANNULUS A HELIUM 0.8m ANNULUS o= FLOW RATE HP/sec.

A AREA n.

-' P=DENS|TY lbs/ft.

o l t l T l 1- O 2 4 6 6 IO [2 CORRELATION OF oxvcsew CONTENT m EFFLUENT o|= COAXIAL JET TORCH WITH PROCESS PARAMETERS;

' INVENTOR.

JOHN E. JACKSON BYQT- Q 54'- ATTORNEY Sept. 1, 1970 J. E. JACKSON METHOD FOR SHIELDING A GAS EFFLUENT Original Filed Jan. '16, 1968 5 Sheets-Sheet cowomows: '/g" cm. NOZZLE INCH STANDOFF l O. 4 O 3 R E 1 Z t 2 a m 1 w m c H l m F 0 l 0 w o m 2 1 M I 0 I O H M 5 2 m w m m m m 0. M P2351 zoififizwozou zu xo kzmzihm /DX INCH" EFFECT ANNULUS WIDTH PARAMETER A ND SHIELDING FLOW RATE 0N SHIELDING PERFORMANCE WlTH A |.O INCH STANDQFF.

ATTORNEY Sept. 1, 1970 J. E. JACKSON 3,

METHOD FOR snmwms A GAS EFFLUENT Original Filed Jan. 16, 1968 5 Shets-Sheet s CONDITIONS a om. uozzus 45o CFH ARC TORCH GAS I50 AMPERES 2 INCH STANDOFF PERCENT IOOO CFH EFFLUENT OXYGEN CONCENTRATION ,o.ooo| |lIl||1|-I1 l 0.25 0.3 0.4 0.5 0.6 as w 1.5 2.0 3.0 4.0

05 INCH INFLUENCE OF SHIELDING FLOW RATE AND ANNULUS WIDTH PARAMETER ON SHIELDING PERFORMANCE WITH A 0.5 INCH STANDOFF.

I N VEN TOR.

' JOHN E. JACKSON FIG. Q f

ATTORNEY United States Patent 3,526,362 METHOD FOR SHIELDING A GAS EFFLUENT John E. Jackson, Indianapolis, Ind., assignor to Union Carbide Corporation, a corporation of New York Original application Jan. 16, 1968, Ser. No. 698,268, now Patent No. 3,470,347, dated Sept. 22, 1969. Divided and this application May 28, 1969, Ser. No. 842,072

Int. Cl. B44d 1/08 U.S. Cl. 239-1 4 Claims ABSTRACT OF THE DISCLOSURE A method wherein a gaseous effluent such as an arc effluent is protected from contamination by its surrounding environment by surrounding such effluent with a coaxial annular stream of gas having a width which is in the range of from 0.25 to about 4.0 times the square root of the diameter of the orifice, measured in inches, in the nozzle, and which has a value for the square root of the momentum flux of greater than 2.0 lb. /sec. ft.

This application is a division of 698,268 filed Ian. 16, 1968, now Pat. No. 3,470,347.

This invention relates to a method for shielding a gas eflluent from the environment around such gas effluent and more particularly to a method wherein such gas eflluent is an arc effluent.

Gas flowing out of a nozzle, hereinafter referred to as gas efiiuent, has been used in many processes particularly in metal fabrication, welding, and coating processes. For example, oxy-fuel gas effluents with and without powder entrainment have been used to cut and treat metals. Gases have been used to shield electric arcs. Gases have also been introduced into electric arcs so that at least some of the gas becomes part of the arc and becomes an arc efiluent. In many cases it is desirable to protect the gas or are effluent from contamination by the natural environment around the gas or are effiuent.

This invention in its broadest aspects relates to a novel method for preventing contamination of a gas effluent by the natural environment around such gas efiiuent. Therefore, it should be understood that the teachings hereinafter set forth are applicable to all processes typified by the above referred to examples, while for sake of simplicity of describing those teachings and presenting a preferred embodiment of the invention the greater part of this specification will refer hereinafter to arc coating processes wherein arc effluents are utilized.

One of the most useful of the arc coating processes is a process wherein an arc is established between two electrodes and a gas is introduced into such are and passed through a nozzle having a constricting orifice. The material to be deposited as a coating, usually in powder form, is introduced into the arc and carried to the workto-be-coated by the arc efl'luent. The process is described in greater detail in U.S. Pat. 3,016,447 issued to R. M. Gage et al.

While this process is eminently useful, there are some undesirable shortcomings. Arc coating or plating, as it is sometimes referred to, suffers from contamination of the arc effluent by the surrounding air. As a result, the coating material becomes oxidized and produces an oxidized coating. When deposits of pure metal are made, the metallic coatings are porous, possess low ductility and are difficult to machine to accurate dimensions with good surface finishes. In addition, the chemical changes which the hot entrained coating material undergo in the presence of the contaminates may alter the desirable physical properties of the coating material.

Accordingly, it is a main object of this invention to provide a method for minimizing contamination of a gas effluent with the surrounding environment.

It is another object to prevent the environment surrounding an arc efliuent from being aspirated into said are eflluent.

Another object is to provide a method for are coating wherein oxidation of the material entrained in an arc eflluent is minimized.

A further object is to provide a method for producing substantially oxygen-free arc coatings on a substrate.

Yet another object is to prevent oxidation of powdered material entrained in an arc efiluent.

These and other objects will either become apparent or will be pointed out when referring to the following description and drawings wherein:

FIG. 1 is a schematic diagram illustrating the concept of the invention;

FIG. 1A is a partial cross section view of the front end of typical torch apparatus for practicing the invention;

FIG. 2 is a curve illustrating the effect of the annular shielding gas stream flow rate on oxygen concentration in the gas efliuent being shielded for various annulus areas and shielding gases;

FIG. 3 is a curve illustrating the effect of the parameters of the annular shielding gas stream on oxygen concentration;

FIGS. 4 and 5 are curves illustrating the effect of the ratio W/D on oxygen concentration at different standoff distances.

The objects of the invention are accomplished in a general way by a method wherein the gas efiiuent, which is to be protected from contamination by its natural environment, is surrounded with an annular shielding gas stream the width of which, measured in inches, should be in the range of from 0.25 to about 4.0 times the square root of the diameter of the orifice (measured in inches) in the nozzle through which said effluent passes. The gas flowing in the annular shielding gas stream is coaxial with the gas eflluent and the square root of the momentum flux of the annular stream is at least 2.0 lbf /sec. ft. as given by the equation (Q/A)(p wherein Q is the flow rate in ft. /sec. of the annular gas stream; A is the annular area in ft. and (p is gas density in lb./ft.

This invention is predicated on the discovery that an annular shielding gas stream having a width and forward momentum flux within limits herein defined and which has uniform turbulent flow will remarkably shield a gas efiiuent so that the oxygen concentration in the gas effluent is essentially equal to that of the annular shielding gas stream. When the width of the annular gas stream is between 0.25 and 4.00 times the square root of the diameter of the orifice in the nozzle from which the gas efiiuent emerges, both measured in inches, and when the square root of the forward momentum flux of such gas stream is greater than 2.0 lbf /sec. ftf as defined by the equation (Q/A)(p the Reynolds number (Ref of the gas flow is usually greater than 2000. While it is possible to practice the invention with a Re less than 2000 the corresponding marginal shielding performance achieved is improved by increasing Re while keeping the ratio W/D within the limits defined.

This invention is admirably suited to and useful for the shielding of an arc effluent wherein a powdered coating material is entrained and carried in a heated state to a workpiece which is to be coated with such material. In the process of arc coating an arc is usually established between a first and second electrode contained in an arc device. An arc gas is introduced into the arc to create an arc effluent. The coating material is introduced into the arc and are effluent and is carried thereby to the work. Up until now the coatings achieved by such a TABLE I.-OXYGEN CONTENT OF ARC TORCH DEPOSITS Oxygen content (percent) Starting Conventional Coaxial jet Coating material powder coating shielded coating 172 .456 .151 Tungstcn 027 274 030 Titanium 655 2. 730 Molybdenum 419 710 160 As will be noted from the table, the oxygen content of deposits made with the invention is substantially and remarkably lower than that obtained with conventional coating, i.e., without coaxial jet shielding, and in most cases (in equal to or even less than the oxygen content in the starting powder.

A schematic representation of typical equipment for carrying out the invention is shown in FIG. 1. Attached to a conventional arc torch device 1 of the type mentioned above and shown in US. Pat. 3,016,447 is a coaxial jet shielding device 3 so that the arc efiiuent from the nozzle orifice having a diameter D in torch 1 passes through the center of the device 3. The coaxial jet shielding device ,3 is selected so that the width measured in inches of the annular shielding gas streams is between 0.25 and 4.00 times the square root of the dimension D measured in inches. In this case the width W of the device 3 is the width of the annular shielding gas stream. In FIG. 1A the diameter D of device 3A is selected and correlated with the diameter D of the nozzle 5A orifice so that the width of the annular gas stream which in this case is falls within the range given below. It should be noted that in this embodiment of the invention, the inner surface 7A of device 3A does not restrict the jet discharging from nozzle orifice 5A but merely provides a passage through which the arc efiluent emerges from device 3A. However, if surface 7A were made sufiiciently small so that substantial constriction of the effluent jet resulted, then the inner diameter of this constriction becomes the orifice diameter for computation of the Width parameter.

FIG. 4 indicates that at a 1 inch standoff distance, that is distance of the arc torch from the work, the effectiveness of the annular shielding gas stream increases for a given flow rate up to a maximum or preferred ratio of about 1.2 in." and then begins to decrease until a ratio of about 4.0 in. is reached at which point the effectiveness is marginal. FIG. 5 is a curve similar to FIG. 4 indicating that the effects illustrated in FIG. 4 are more pronounced when the standoff distance is /2 inch. It has been found that the invention herein described is useful in processes where the standoff distance, measured from the end of the device, is up to about 40 to 50 times the nozzle orifice diameter.

The curve of FIG. 2 illusrtates that the molecular weight of the gas used has an effect on the resultant oxygen contamination of the arc eflluent. It is evident that argon is more effective than helium and that at the same flow rate, it is more desirable to use a smaller annulus and achieve higher velocities.

In addition to being dependent on the width of the annular shielding gas stream, successful shielding performance according to the invention is dependent on what is called herein the square root of the momen um flux 4 and which I chose to define by the equation (Q/A) p The derivation of the term (Q/A f is as follows:

If the term (Q/A) were written as (Q /A r or as one ends up with momentum flux to the half power since (Q is the weight flow rate. (Q/A) is the velocity and l/A) is the area to the minus 1 power. In English units the weight density is divided by 32.2 to give a mass density.

Under these conditions (Q/A) r of .1/2 sec. ft."

is equal to slugs 0352 see. ft.

In FIG. 1 the area A is the cross sectional area of the annulus through which the shielding gas passes. In FIG. 1A the area is computed by 17/4 [D -D FIG. 3 illustrates that in order to achieve a significant reduction in oxygen content in the arc efliuent the value of (Q/A) should be greater than 2 lb. /sec. ftf

The composition of the gas, of course, effects the momentum flux. FIG. 2 indicates that argon gas for example is a better shielding gas for purposes of this invention than is helium.

In a preferred embodiment of the invention, an arc torch having a tungsten electrode and nozzle with an /8 in. orifice diameter was adapted to receive a coaxial jet shielding device of the type shown in FIG. 1A having a diameter at D of 1 inch. The annulus in the shielding device is preferably covered with porous metal, screens or other material to insure uniform flow around the cross section without velocity disturbances caused by the gas inlets. The invention is most useful when uniform turbulent flow is achieved as opposed to smooth laminar flow. An arc is established between the tungsten electrode and a second electrode in the arc torch. Powdered coating material is introduced into the arc and carried to a workpiece in the arc eflluent. Shielding gas, preferably argon, is introduced into the coaxial jet shielding device 3 or 3A to produce a flow rate capable of providing the necessary momentum flux.

If the coaxial jet shielding device has an annulus which is too wide, very high flow rates are needed to achieve the desired momentum flux and the process becomes less attractive commercially. On the other hand, if the annulus is too narrow, the annular shielding gas stream becomes too thin and unstable and does not perform its function of reducing the amount of gas entrained by the central gas effluent. I have also found that the are conditions such as are amperage and are gas flow rate have a negligible effeet on the effectiveness of the annular shielding gas stream. For example, the arc gas was varied from 200 c.f.h. to 600 c.f.h. with very little change in the oxygen content in the arc effluent.

Having described the invention, the following examples are illustrative of conditions falling within the scope of the invention and are provided to assist those skilled in the art in understanding how to practice the inventive concept.

EXAMPLE I An arc torch having a tungsten electrode surrounded by a nozzle having /8 in. diameter and W in. length was used as the coating device. Argon gas was introduced into the arc torch as an arc gas at a flow of 450 c.f.h. The are current was amperes at 78 volts. The torch standoff was /2 in. Nickel powder having an oxygen analysis of .172% was introduced into the arc torch at the rate of 24 g.p.m. (grams per minute). The rate of deposition of the powder on the substrate was 15 g.p.m. A coaxial jet shielding device similar to that shown in FIG. 1A was attached to the arch torch. The annular coaxial gas stream surrounding the arc efiiuent had a width of 0.312 in. Argon gas was introduced into the device at a flow rate of 2000 c.f.h. to produce a value for the square root of the momentum flux of The oxygen analysis of the coating formed was .150%, a reduction of .022%. The coating was very clean with excellent machinability.

EXAMPLE II Apparatus similar to that used in Example I was used in this test with the exception that the nozzle length was A; in. Argon gas was introduced ioto the arc torch at a flow rate of 450 c.f.h. The are current was 100 amperes at 80 volts. The standoff distance was /2 in. Titanium powder having an oxygen analysis of .651% was introduced into the arc torch at the rate of 20 g.p.m. The rate of deposition of the powder on the substrate was 13 g.p.m. The coaxial jet had a width of 0.312 in. Argon was introduced into the coaxial jet at the flow rate of 2000 c.f.h. to produce a value for the square root of the momentum flux of The oxygen analysis of the coating formed was .730%. The machinering characteristics and ability to take a high surface finish was outstanding.

EXAMPLE III The oxygen analysis of the coating was .155. The reduc tion in oxygen was a remarkable 2.64%.

EXAMPLE IV The conditions here were the same as in Example III. The are current was 150 amperes at 80 volts. Tungsten powder having an analysis of .027 oxygen was introduced at the rate of .68 g.p.m. into the arc torch. The deposition 6 rate was 51 g.p.m. The width of the coaxial jet was 0.312 in. and its square root of the momentum flux was sec. ft." 1 The oxygen analysis of the coating was 030%. The coating retained Substantially the low oxygen level of the original tungsten powder making it more suitable for applications requiring high strength at room and high nular coaxial gas stream the width lot which meas-.

ured in inches is in the range of from 0.25 to about 4.0 times the square root of the diameter of the orifice measured in inches, in said nozzle; introducing gas into said annular coaxial gas stream at a flow rate to cause said anmfi lar coaxial gas stream to have a value for the sqiiare root of the longitudinal momentum fiux that is greater than 2.0 lb. sec. ft. as given by the equation (Q/AM wherein Q is the flowrate in ti /sec. of the annular gas stream; A is the annular area in :ft? and is gas density in lb./ft. i 2. Method according to claim 1 wherein the width of the annular coaxial gas stream is about 1.2 times the square root of the diameter of the orifice in said nozzle when both are measured in inches. 3 j

Method according to claim 1 wherein the Reynolds number of the coaxial gas stream is greater than 2000. 4. Method according to claim 1 wherein the gas in the annular coaxial gas stream is argon.

References Cited UNITED STATES PATENTS 42/1967 Unger at al. 219-46 9/1969 Jackson 219-76 US. Cl. X.R. 

